Changing plan mid-month now does the fair thing in both directions

Two different situations were being treated as one.

Moving up happens immediately. You asked for more capacity because you need it now, so you get it now. We charge only the difference for the rest of the period, with credit for the time you had already paid for on the old plan, and your renewal date does not move.

Moving down waits until the period you have paid for ends. You bought this month at the higher tier, so you keep it: nothing is removed early, nothing is charged today, and there is no refund to chase. Your billing screen shows the date the change lands and what will be over the new plan's limits when it does - in your own numbers, before you confirm rather than a month later. You can call the change off at any point before the date.

Switching from annual to monthly follows the same rule, because a year has already been paid for. Switching plan during a trial charges nothing and never shortens the trial.

Billing is now five tabs rather than one long page - your plan, usage, change plan, discounts, and payment and invoices - and the plan comparison shows what each tier actually includes instead of only its price.